1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the same. More specifically, it relates to a semiconductor device including an intralayer lens which is suitably used in solid image pickup devices such as CCDs (charge-coupled devices) and liquid crystal display devices, and a method of manufacturing the same.
2. Description of Related Art
Solid image pickup devices of CCD or MOS (metal oxide semiconductor) type have been used in various applications such as digital cameras, video cameras, cellular phones equipped with cameras, scanners, digital copiers, facsimiles and the like. As the application becomes wider, there are increasing demands for higher performance and function such as increase in pixel number and improvement in light receiving sensitivity, as well as reduction in size and cost.
As the size reduction and the pixel number increase progress in the solid image pickup device, the size of pixels installed in the solid image pickup device becomes smaller. This decreases the light receiving sensitivity, which is a fundamental performance, and may possibly cause difficulty in clear imaging under a certain light level.
To deal with such a problem, was adopted a technique of forming a microlens of an organic polymer material on a color filter to increase the light receiving sensitivity. However, this is no longer sufficient to achieve a satisfactory effect (for example, see Japanese Unexamined Patent Publication No. HEI 4 (1992)-12568). Accordingly, a technique of forming a lens within a layered structure, i.e., a so-called intralayer lens disposed between a color filter and a light receiving part below the color filter, has been adopted in combination with the microlens technique.
FIG. 1 shows a schematic sectional view of a single pixel (unit cell) of a CCD solid image pickup device including a prior art intralayer lens (see Japanese Unexamined Patent Publication No. HEI 11 (1999)-40787). FIGS. 2(a) to 2(e) illustrate an example of a method of manufacturing the prior art intralayer lens of FIG. 1.
First, as shown in FIG. 2(a), a semiconductor substrate 1 is subjected to ion implantation of required impurities to form a light receiving part 2, a read-out gate 3, a CCD transfer channel (transfer part) 4 and a channel stopper 5. Then, a transfer electrode 7 of a predetermined pattern is formed on the surface with the intervention of an insulating film 6 and a light shield film 9 covering the transfer electrode 7 is formed with the intervention of an interlayer insulating film 8. The light shield film 9 is patterned to have an opening positioned above the light receiving part 2.
Then, referring to FIG. 2(b), a film formed by reflowing such as a BPSG (boro-phosphosilicate glass) film or a film formed by plasma CVD (chemical vapor deposition) is provided on the light shield film 9 to form a first flattening film 10 to flatten the surface.
On the first flattening film 10, a material layer 16 for forming an intralayer lens 11 having a high refractive index (e.g., a silicon nitride film) is formed by plasma CVD as shown in FIG. 2(c).
Then, as shown in FIG. 2(d), a photoresist 17 is applied onto the lens material layer 16 and patterned into the shape of a desired intralayer lens 11, followed by reflowing at about 160° C.
Then, referring to FIG. 2(e), the lens shape of the photoresist 17 is transferred to the lens material layer 16 by dry etching to form the intralayer lens 11.
Thereafter, a second flattening layer 12 (not shown) covering the intralayer lens 11 is formed to flatten the surface. Then, a color filter 13, a protective film 14 and a microlens 15, which are not shown, are formed in sequence. Thereby, the CCD solid image pickup device shown in FIG. 1 is obtained.
In the above-described step of transferring the intralayer lens by dry etching using the lens-shaped resist as a mask, however, the following problems are caused.
First, upon forming the resist into the lens shape, the resist is generally heated to about 160° C. on a hot plate for transformation until its surface tension and interface energy with respect to the underlying intralayer lens material layer reach a state of equilibrium. If gaps between adjacent patterned resists are filled to connect the resists upon fusing under heating, the transformation of the resist continues until a stable shape is obtained. This makes difficult to obtain a predetermined lens shape.
Second, upon dry etching using the lens-shaped resist as a mask, it is difficult to obtain the intralayer lenses of uniform shape due to nonuniformity in lens shape of the resist and variation in etching speed. Further, because of the limited etching selectivity between the resist and the intralayer lens material layer, the choice of materials is limited.